1. Field of the Invention
The present invention relates to an apparatus for calculating a state of charge, and more particularly to a technique of calculating a state of charge of a power storage system provided with a first power storage device and a second power storage device.
2. Description of the Background Art
Conventionally, an electric vehicle equipped with an electric motor as a driving source is known. Such a vehicle is equipped with a power storage device that stores electric power, such as a battery and a capacitor. Electric power stored in the power storage device is supplied to the electric motor. The electric motor is controlled so as to output desired power.
One problem with the electric vehicle lies in that the distance that the vehicle can run on a single charge is shorter than in a vehicle having an internal combustion engine as a driving source. One solution for extending the distance that the vehicle can run is to increase the total capacity of the vehicle such as by mounting thereon a plurality of power storage devices, for example.
Another problem with the electric vehicle is to protect a power storage device against overdischarging and overcharging. For example, when the state of charge of the power storage device is low, output power of the electric motor is preferably limited so as to avoid overdischarging. When the state of charge of the power storage device is high, electric power to be charged into the power storage device is preferably limited so as to avoid overcharging. To suitably limit the output power of the electric motor and the electric power to be charged into the power storage device, the state of charge of the power storage device needs to be monitored.
Japanese Patent Laying-Open No. 2006-112786 discloses a state-of-charge detection method of computing the state of charge of a battery as a first state of charge based on integration of current, computing the state of charge of the battery as a second state of charge based on voltage, and computing a synthesized state of charge obtained by a weighted average of the first state of charge and the second state of charge, as a state of charge of the battery.
However, the state-of-charge detection method disclosed in Japanese Patent Laying-Open No. 2006-112786 is intended to calculate the state of charge of a single power storage device, rather than calculating the total state of charge of a power storage system having a plurality of power storage devices. Assuming that a simple total sum of the states of charge of the respective power storage devices is calculated as the total state of charge of the power storage system, the following problem may arise.
For example, in the case where the state of charge of one power storage device is 90% and the state of charge of another power storage device is 60%, electric power to be charged needs to be limited as compared to the case where the states of charge of the two power storage devices are both 75%, even though the total state of charge of the power storage system is the same in both the cases. However, calculating a simple total sum of the states of charge of the respective power storage devices as the total state of charge of the power storage system will make it difficult to distinguish between the case where the respective power storage devices have different states of charge and the case where the two power storage devices have an equal state of charge. It may therefore be difficult to determine that electric power to be charged needs to be limited.
Similarly, in the case where the state of charge of one power storage device is 10% and the state of charge of another power storage device is 40%, electric power to be discharged needs to be limited as compared to the case where the states of charge of the two power storage devices are both 25%, even though the total state of charge of the power storage system is the same in both the cases. However, calculating a simple total sum of the states of charge of the respective power storage devices as the total state of charge of the power storage system will make it difficult to distinguish between the case where the respective power storage devices have different states of charge and the case where the two power storage devices have an equal state of charge. It may therefore be difficult to determine that electric power to be discharged needs to be limited.